The roll grooving of metal pipe, and the use of such roll-grooved pipe in conjunction with segmented pipe couplings is well-known in the art.
Roll-grooving was developed to enable the use of segmented pipe couplings on lightwall pipe which has too thin a wall thickness for it to be "cut grooved". Roll grooved lightwall pipe, is often less expensive than standard weight cut grooved pipe, particularly when using stainless steel.
The rolled groove profile originally developed was similar to the familiar cut groove of the day. It was essentially a square groove with some necessary minor profile modifications added to maintain adequate joint strength. These modifications included radiused corners at the bottom of the groove for reasons of pipe strength and formability, and a slightly narrower width to reduce angular deflection of the joint for joint strength.
Grooves of this profile have served the general industry well for some 35 plus years and will continue to do so.
A typical segmented pipe coupling is those disclosed in U.S. Pat. No. 4,611,839 Rung et al. issued Sep. 16, 1986. Also, typical of an apparatus for use in the roll-grooving of pipe is the one disclosed in U.S. Pat. No. 3,995,466, Kunsman, issued Dec. 7, 1976.
In the roll-grooving of metal pipe, the pipe is supported on rollers, and a groove forming tool is affixed to one end of the pipe on a radius thereof. The groove-forming tool has complementary power driven mutually presented groove-forming rollers that are forced towards each other, and which, in the process, impinge on the inner and outer circumferences of the pipe at the said radius. The driven rolls then progressively deform the pipe under cold working to provide a continuous circumferential groove in the outer circumference of the pipe, the groove being suitably configured to accommodate the keys of a segmented pipe coupling. During this groove-forming operation, the pipe rotates on its supporting rollers, the groove-forming rollers being held in fixed axial position by a supporting frame. Alternatively, the pipe can be held fixed, and the groove forming rollers be caused to orbit the pipe end.
Prior to the present invention, the rolled groove has been essentially rectangular in transverse cross-section, for the groove to provide an axially extending bottom wall flanked by mutually presented radially extending side walls that extend essentially perpendicular to the pipe axis.
The rolling of the groove requires that the material of the pipe wall in the location of the groove, be displaced radially inwardly of the pipe. This results in the formation of a step in the bore of the pipe. That step, under hydrodynamic or aerodynamic conditions produces eddy currents and turbulence in the conveyed fluid. While this generally results in negligible pressure loss, under severe conditions of high velocity or abrasive flowing media, it can cause increased wear at the step in the joint and immediately downstream thereof.